BACKGROUND: Lysozyme is an innate immune peptide with bactericidal and fungicidal activity (FA). Despite increased expression of lysozyme protein in chronic rhinosinusitis (CRS) sinus mucosa, CRS patients experience repeated bacterial and/or fungal infections. Commercial sinus irrigation solutions are often used to provide symptomatic relief. However, one of the mechanisms of action of lysozyme involves ionic interactions with the microbial cell wall, which may be inhibited by ionic solutions such as commercial sinus irrigation solutions. OBJECTIVE: Determine if the FA of lysozyme is reduced in the presence of solutions with increasing ionic strength and inhibited in the presence of commercial sinus irrigation solutions. METHODS: Using an in vitro colony-forming unit (CFU) assay, the FA of lysozyme (5 μM) was tested against a fungi commonly isolated from CRS patients, Aspergillus fumigatus, in solutions of increasing ionic strength or commercial sinus irrigation solutions. FA was presented as percent of control. RESULTS: FA of lysozyme against A. fumigatus was 95% in a 21-mM ionic strength solution. However, with increasing ionic strength, FA decreased and was abolished in a 46-mM ionic strength solution. Commercial sinus irrigation solutions abolished the FA of lysozyme against A. fumigatus. CONCLUSION: The in vitro FA of lysozyme is dependent on the ionic strength of the solution. The use of sinus irrigation solutions should be further evaluated with regard to maintaining functional activity of cationic antimicrobial peptides involved in sinonasal innate immunity.